Manufacture of heat insulation structures



May 16, 1944.

R. A. M RTHuR EI'AL MANUFACTURE OF HEAT INSULATION STRUCTURE Filed July 20. 1939 INVENTOR ATTORNEYS Patented May 16, 1944 MANUFACTURE OI HEAT INSULATION STRUCTURES Roger A. MacArthur and Harold W. Greider, Wyoming, Ohio. assignors toffl e Philip Carey Manufacturing Company, acorfioration of Ohio Application July 20, 1939, Serial No. 285.544

11 Claims.

This invention relates to the method of producing heat insulation structures. It relates especially to the method of producing preformed structures comprising formedbodies of heat insulation materials.

Heat insulation materials, because of their relatively low conductivity of heat, are used to decreas the transmission of heat energy by conduction or radiation either to or from a particular body. The use of heat insulationto cover boilers, steampiping, etc., thus conserving fuel. is well known. No substance is perfectly eflicient as a heat insulation material as all substances conduct heat to a certain extent. Certain materials are, however, more eflicient than others. Also the efllciency of a particular material varies with the temperature at which it is used. Those materials having relatively low heat conductivity are known as heat insulation materials and are recognized as a class distinct from ordinary structural materials. Some heat insulation materials otherwise efllcient cannot endure high temperatures and such materials are referred to as low temperature heat insulation materials. Those heat insulation materials which are adapted to resist relatively high temperatures are known as high temperature heat insulation materials.

It is a purpose of this invention to produce a heat insulation structure which is very high in heat insulation efliciency and which is adapted for use in insulating surface that require a high temperature heat insulation in order to withstand the temperature at which the surface to be protected is maintained.

It is a further purpose of this invention to manufacture a preformed structure of the high heat insulation efliciency referred to that can be installed very economically and that can be manufactured by economical production methods.

Heretofore heat insulations have been made by molding slabs of substantial thickness in the form of flat blocks, curved sections suitable for pipe coverin s, and the like, from various compositions that can be formed to desired shape and dimensions and that contain heat insulatin and heat-resistant ingredients. The compositions used are adapted to become bonded after formation to desired sha e and dimen ions so t at they will have suflicient strength to stan up in use. One type of heat insulation mater a that has been extensively manufactured and used is that which is commonly referred to as 85% magnesia, 85% magnesia insulation has heretofore been produced by rapidly heating a 2 to 3% solution of magnesium bicarbonate to above about 200 F., to precipitate basic magnesium carbonate. The basic magnesium carbonate thus produced is then incorporated in a slurry, the solid components of which consist of about of. basic magnesium carbonate and about 15% of asbestos fiber. The slurry is molded in filter molds of desired shape under a pressure of about 15 to 40 pounds per square inch and the molded articles are thereafta dried and trimmed to desired dimensions. 85% magnesia insulation is suitable for insulating surfaces which are maintained at about 550 F., or less. If the insulation were to be used at temperatures above about 550 R, the basic magnesium carbonate would tend to decompose with resultant injury to the physical integrity of the insulation. Since 85% magnesia heat insulation material is very extensively used and is ill suited for use at temperatures above about 550 F., insulations of this type, namely. insulations which are particularly suited for use at temperatures below about 550 F., have become known as low temperature heat insulation materials. Heat insulation materials which are adapted for use at temperatures above about 550 E, are commonly referred to as high temperature heat insulation materials.

Heat insulation materials of the character referred to, both high and low temperature insulations, have been made heretofore in the form of molded slabs of substantially uniform characteristics throughout and the slabs such as blocks. pipe coverings and the like have been sold in separate units of standard dimensions for installation on the job.

It is a feature of the present invention that a preformed heat insulation structure appropriate for sale in the form of units of desired size and shape comprises at least two slabs of heat insulation material having different heat resistance characteristics and is manufactured by first making in any suitable way a slab of heat insulation material of desired dimensions and then molding an aqueous slurry in integral contact with the previously formed slab, which slurry is adapted to become set by chemical action to form another slab integrally bonded with the first slab in a composite heat insulation structure. The setting of the aqueous slurry by chemical action, may. for example, be the result of hydraulic setting after the manner of the setting of Portland cement. or may be the result of application of heat. Heat insulation compositions which can be set by chemical action a a result of heating or hydraulic action are advantageous since such compositions do not shrink to a substantial extent during drying. Such shrinkage, if it occurs. tends to impair the bond between the slabs and results in distortion of the desired dimension oi the composite insulation. Preferably the slurry, that is molded integrally with the slab first formed, comprises normal magnesium carbonate, and the slurry is adapted to become set by heating the molded slurry while moist to convert the normal magnesium carbonate therein to basic magnesium carbonate. when the slurry.

that is integrally molded with a previously formed slab, contains normal magnesium carbonate, and such molded slurry is set by heating the slurry to convert the normal magnesium carbonate to basic magnesium carbonate, the Juneture between the slabs is characterized by the cemented relationship of the basic magnesium carbonate thus formed in situ with the surface of the heat insulation material of the other slab and the manufacture of such products constitutes one of the objects and features of this invention. .It is advantageous in the practice of this invention that the first formed slab of heat insulation material be formed by forcing an aqueous slurry of heat insulation ingredients into a filter mold with drainage of water from the mold, the material thus formed being removed from the mold, and dried and trimmed to desired dimensions before molding the second slab of heat insulation material integrally therewith. It is apparent from the foregoing that in the practice of this invention desirable unitary heat insulation structures are ailorded wherein there are at least two layers or slabs that differ in heat resistance characteristics. One of the layers or slabs can be appropriate for use at low temperatures while the other is adapted for use at higher temperatures, each of the layers being permanently bonded to the other in the form of a preformed structural unit.

In order that a clear understanding may be had of this invention, it will be described hereinbelow in connection with certain specific embodiments thereof which are shown in the accompanying drawing, wherein Figure 1 is a perspective view of a preformed heat insulation structure according to this invention, the two layers of heat insulation material being offset relative to each other;

Figure 2 is a perspective view on a somewhat smaller scale of the preformed heat insulation structure wherein the two layers of heat insulation material are not offset;

Figure 3 is a transverse sectional elevation of a portion of the heat insulation material shown in Figs. 1 and 2:

Figure 4 is a perspective view of a heat insulation structure embodying this invention in a form suitable for a pipe covering with the two layers of heat insulation material oflset relatively to each other:

Figure 5 is a transverse sectional elevation of a portion of a heat insulation structure showing supplementary mean which may be used in a structure according to this invention for securing the layers of heat insulation material togather: and

Figure 6 is a horizontal sectional view of a mold which may be used in the manufacture of a heat insulation structure according to this invention.

In the embodiment of this invention shown in Figs. 1,2and3theheatinsulationmaterial comprises two layers or slabs of heat insulation material. These layers are indicated by the reference characters l0 and l I. The heat insulation characteristics of the layer iii are substantially different from the heat insulation characteristics of the layer H, as will be brought out more in detail below in connection with illustrative examples of heat insulation materials that may be used in these layers. The heat resistance of one of the layers is substantially greater than the heat resistance of the other layer. By heat resistance reference is made to the capacity of the heat insulation material to resist elevated temperatures without substantial decomposition or other change which results in excessive weakening of the molded heat insulation material. In this particular embodiment of this invention the layers Iii and II are integrally bonded with each other by virtue of the fact that one of the layers has been molded in contact with a surface of the other layer in a manner that will be described more in detail below.

In Fig. 1 the layer III is oflset with reference to the layer ll along two non-parallel edges. although the marginal dimensions of the two layers are the same. In other words, the structure is provided with complementary oifset portions. both at the ends and at the sides. The structure shown in Fig. 2 is similar to the structure shown in Fig. 1, except that the layers It and i I are not offset with respect to each other.

In the embodiment shown in Figs. 1. 2 and 3. the layers ii and I! may conveniently be about 1 V inches in thickness, for example. The length and width of the layers ll and H may be of any desired dimension. When the length is about a foot or more, the oflset at the ends shown in Fig. 1 may be conveniently about 1 or 2 inches. The oii'set along the sides may likewise be about 1 to 2 inches or somewhat less. It is apparent. however, that the thickness of the different layers and the dimensions of the component parts of the structure and the extent of overlap or offset, if any. of the layers may be varied as desired.

In Fi 4 the insulation structure is curved so as to be appropriate for a pipe covering and is composed of two layers '2 and I. of heat insulation material. The heat insulation characteristics of the layer it differ from the heat insulation characteristics of the layer It, as will be r 'ierred to more in detail below. The structure smown in Fig. 4 may be of any desired dimensions. For fitting snugly about a 3 inch pipe, for example. the layer I! may conveniently be about 1 inch in thickness and the layer ll may conveniently be about 1% inches in thickness. In a pipe covering of these dimensions the offset of the layers l2 and I! may be about V4 to A inch along the sides and about 1 to 2 inches at the ends. Insulation structures suitable for pin coverings may. of course, be made with no offset whatever. Moreover, insulation structures of other shapes, either flat or curved and of any desired size and dimensions may be made embodying this invention.

In order to illustrate one embodiment of this invention, the constituents oi the component parts thereof will be described in connection with an illustrative example of a method of manufacturing a heat insulation structure according to this invention. A formed layer or slab of a heat insulation material especially appropriate for re-- sisting high temperatures. that is, temperatures such"asj aboutfllifiiilfill' tii'about 1200' it. may be made as in U. 8. Patent No. 2,042,098 issued im-ilgufl d l w mwmca tion describedfinthis patent. an aqueous slurry solids about 30% of basic drate'd lime. about 3% of dip I and the balance-inert material such u-eoiommpunues, etc. The basic magnesium carbonate is iormed by rapidly heating a 25b! "8% solutioii', 'oifmagneslum carbonate to above about 2Q0 I". one calcium carbonate is precipitated treating a calcium hydroxide suspensionywith arbon dioxide. The slurry containingthe ingredients above mentioned is molded in affilterfmn "undera pressure of about 50 poundsjper squa rich to'iorm a molded articie whifihf'aft'er ding; is dried; The molded article may correspondwith the slab il shown in FigsYLl-Z or 3dr j'slab i3 shown in Fig. 4. After dryingithe ornied slab is trimmed so as to be ofthe dimenslbnsde'sired for the completed structure. v i

fi'he slab ofhigi'i temperature heat insulation material which' is or the desired shape and dimensions is-(placedina mold which is larger than the slab oi lightenrperature heat insulation material tha is'inserted therein and which is suitableuoi molding a low temperature heat insulation"'integially with bne side of the slab'of high tem erature": heatdnsulation. For example, in makinga-structuresuch as that shown in Fig. 2, a simple mold such as-the mold shown in Fig. dmay be'use'dfi "lhefmoldmay be made in two parts-timid lithat are adapted to be detach.- ably' secured together' by any suitable means, not shownifl'he'nioldis adapted'to receive the 'previously iormedf'fslabiii or high temperature heat insulation 'coniposition. The low temperature heat insulation coni'position is then introduced illtothe v cant bltioi'rlb of the mold thereby molding the wtemperature heat insulation compo'si nintegrallywiththe previously formed slab of high temperature heat insulation material. The layer hu's inolded is the layer ill shown in Fi sz 'L 2 andj-3 Y Onepreferred ype oflow temperature heat insul'atioir ior" employment in combination with a high temperaturi' lieat insulation material according o' 'this i'r'ivention may be madeby preparlng ari' adueous slurry containing as solids "about?89%"- ofa-finelwdivided normal magnesium carbonate crystals and -about 11% oi asbestos fiber; :-ThisFslurry:-is1 -poured into themold in a .seml fiuid conditionfi-tl'iere being present about 3.5 parts of water to each part of solids.- The heatr msulation 'material; after being molded in contact with fsuriace 'of the previously formed :sl'ab oi higWtenzperatureheat insulation material can s setl'imerely by heating the molded composition ficontaining normal magnesium carbonateitcrystalssto a temperature above about "1609x1 2, andi 'preierably to about 185 F. or more, .to": convert i' theiii normal magnesium *carbonate :crystals to basic-magnesium carbonate. -'The resulting heat' insulation: material molded integral- ;lymwithitheipreviouslynformed :high temperature heat-:insulation? material is a low temperature heat insulation corresponding to the 85% magmesiaririsdlatibnhereinabove referred to and has rhighithermalcheat Finsulating eiilciency at tem- ;1perature s;;belomabout550 B, The formation 01' the 85% magnesia insulation material from a assasas slurry containing normal magnesium carbonate crystals is to be preferred to forming the basic magnesium carbonate prior to molding. inasmuch as a composition containing normal magnesium carbonate can be set by heat and dried without substantial shrinkage oi the molded material. Moreover, such a mass will harden to form a slab having adequate strength for most purposes even though no molding pressure is used and even though the composition is not molded-in a filter mold. One can, or course, in the practice or this invention, use a filter mold in molding the low temperature heat insulation composition especially when an excess or water is present and the molding may be under pressure.

In the manner above described a low temperature heat insulation material having desired dimensions for the finished heat insulation structure can be molded without necessarily resorting to pressure molding and without deformation or weakening of the bond between the two magnesium carbonate have preferred characteristics for use according to this invention, both from the point of view of the method of manuiacture and from the point of view of the finished product.

When the composite structure is to be used as a pipe covering, the inner layer or slab i3 ordinarily is of high temperature heat insulation material and is formed first. Thereafter the layer. 52 of low temperature heat insulation is integrally molded with the outer surface of the layer is. 1

In making a composite heat insulation structure embodying this invention, various other heat insulation materials than those above described for purposes of exemplification may be used. For example, instead of the high temperature heat insulation material described above, the heat insulation may be made consisting primarily of porous silica. The silica can be bonded together either by heat in the presence of a suitable flux or by means of a binder. In such case it is preferable to make the porous silica block and then mold the low temperature insulation integrally with one surface of the porous silica block.

In the practice of this invention, it is preferable to first form the high temperature heat insulation material, and thereafter mold the low temperature heat insulation material integrally therewith. However, it is possible to first form the low temperature heat insulation material and thereafter mold the high temperature heat insulation material integrally therewith. For example, a block-or slab of ordinary magnesia insulation may be made in the manner hereinabove described using basic magnesium carbonate precipitated from a magnesium bicarbonate solution. the insulation being molded in a filter mold under pressure with expulsio o wcte and thereafter P dried and trimmed. Aiterthe block or slab of low temperature heat insulation material of desired dimensions has been made. a high temperature heat insulation material can be" molded inteof normal magnesium carbonate, about 42 parts of calcium carbonate. and 13 parts of asbestos fiber and containing sufiicient water to enable the slurry to readily fiow into the mold. After the slurry has been molded integrally with the previously-i'ormed slab or block of low temperature heat insulation material, the molded high temperature heat insulation composition can be hardened or set by heating the composite structure, while the molded high temperature heat insulation composition is moist, to above about 180 F., and preferably to about 185 F. (or above), to convert the normal magnesium carbonate to basic magnesium carbonate. The heat insulation thus prepared is suitable for resisting higher temperatures than the temperatures which can be resisted by the 85% magnesia insulation forming part of the structure. The structure as a whole is suitable for use when the high temperature insulation layer of the composite structure is placed in contact with a surface maintained at temperatures ranging from about 550 F., to about l200 l".

The advantage of a structure such as that above described is believed to be apparent. When the composite structure is positioned on a surface to be insulated, which surface is to.be maintained at a temperature of about 550 1!, to about 1200 F., the high temperature insulation layer of the composite structure is placed in closest proximity to the surface and the layer of low temperature insulation material is on the outside. The low temperature insulation of the outer layer has greater heat insulation emciency at lower temperatures than does the high temperature insulation oi the inner layer. Moreover, since the inner layer oi high temperature insulation protects the outer layer of low temperature insulation, the outer layer of low temperature insulation can be composed of materials which are not as resistant to impairment by heat as are the ingredients of the high temperature insulation of the inner layer. In this way a product is aflorded which has a maximum degree of heat insulation emciency while at the same time being adapted to resist the heat to which the component parts of the structure are subjected in service.

A product having the high degree of heat insulation eifici ency above referred to can be installed in a very economical manner. In order to achieve an equally high heat insulation eiiiciency using heat insulation materials at the present time on the market, it would be necessary to make high temperature heat insulation shapes and to separately make low temperature heat insulation shapes and first install the high temperature heat insulation and follow such insulation by asecond installation of an overlying layer of low temperature heat insulation. The composite structure of this invention can, on the other hand, be installed cost is,

drying and do not tend to disrupt the bond between the two layers of heat insulation material during setting and drying.

Hereinabove, a low temperature heat insulation composition has been described that is adapted to set upon application of heat. A low temperature heat insulation composition that sets hydraulically may be made, using a mixture of gypsum and diatomaceous earth, For example, an insulation composition may be made up in the form of an aqueous slurry containing as solids about 75% of diatomaceous earth and about 25% of gypsum. The slurry thus made will set hydraulically and may be used as the low temperature heat-insulation of a composite structure embodying this invention.

If the shape or slab of the composite structure which is first made is a low temperature heat insulation material, this portion of the structure may be made, using mineral wool together with some suitable binder such as starch, sodium silicate, asphalt or clay. Such a low temperature heat insulation may be made up containing a major proportion of mineral wool and a minor proportion of binder and then molded under slight pressure and dried.

When the low temperature heat insulation material is formed first, a hydraulically setting high temperature heat insulation material may be used therewith, such as a mixture containing about 25% Portland cement, about 70% diatomaceous earth, and about 5% of asbestos fiber. The mixture may be made up as a typical aqueous cement mix or slurry and then molded integraily with the previously formed block or slab of low temperature heat insulation material.

Another composition which is especially suitable for high temperature heat insulation and which sets hydraulically may be made up by using about of diatomaceous earth, about 40% of a high alumina fused hydraulic cement such as "Lumnite," and about 10% of an asbestos fiber.

Eidoliated mica may be used in the low temperature heat insulation component of the structure as by using it with a suitable proportion of Portland cement binder. A mixture containing about 30% of exfoliated mica, about 10% of asbestos fiber, and about of Portland cement, is suitable.

Another example of a high temperature insulation material which is adapted to be set by heating consists principally of a mixture of calcium oxide and calcined diatomaceous earth. For example, a material containing about 30% of calcium oxide, about of calcined diatomaceous earth and about 5% of asbestos iiber may be molded in the form of a semi-plastic slurry. Upon heating the mold under steam pressure in a closed chamber the lime reacts with the diatomaceous earth to form calcium silicate which no: as a bonding agent for the insulation mate- While several examples of heat insulation materials have been mentioned which are adapted to set either by heating or hydraulically, it is not essential that these materials be molded integrally with another layer of heat insulation material previously formed. Any of these insula-- tions may constitute the layer which is formed initially. a

As shown in Figs. 1 and 4, it is usually preferable to have the layers or slabs of different heat insulation materials oifset relatively to each other. when the units of the heat insulated structure are constructed in this manner, they in can be installed in such a way as to minimize the occurrence of cracks extending from the outside of the insulation structure through the insulation to the surface to be protected. Moreover, the overlapplns or offsetting of the layers is assists in maintaining the units of the composite structure in place after they are installed. when curved insulation units are made, e. g., as shown in Fig. 4, and the layers or slabs of insulation materials are offset relatively to each other along 2. the side edges thereof, it is preferable to construct the insulation so that the surfaces of the layers adapted for complementary positioning with another similarly-formed insulation unit will be straight rather than curved throughout the portions thereof that extend beyond the diameter oi the insulation that approximately bisects the offset portions of layers.

When it is desired to bond the layers or slabs of heat insulation material very strongly to each 3 other, one of the layers can be formed first and mechanical securing means may be driven therethrough so as to extend from the opposite side. Thereafter, the other layer of the composite structure is molded so as to surround the ends of the securing means protruding from the slab or layer first formed and so as to be molded integrally with the surface of the slab or layer first formed. In this way a very strong and permanent bonding between the two layers or slabs 02 an the composite structure can be obtained. Thus, in making a structure such as that shown in Fig. 4, the slab i2 may be formed and staples id driven therethrough, the ends of which become embedded in the slab i3, as shown in Fig. 5, when the slab i3 is molded integrally with slab 92.

While this invention has been described in connection with certain specific embodiments thereof, it is to be understood that this has been done merely for the purpose of exempliflcation and that insulations embodying our inventive idea may be varied widely without departing from the scope of this invention as defined by the language of the following claims.

We claim:

1. A method of making a heat insulation which comprises making a slab of high temperature heat insulation material in a filter mold by forcing an aqueous slurry of solid heat-insulation ingredients into the mold with drainage of water from the mold, removing the formed slab from the filter mold, then trimming and drying said slab, making an aqueous slurry comprising normal magnesium carbonate and mineral fibres, molding said slurry in the form of a slab in integral .contact with one surface of said slab of high temperature heat insulation material and then heating the resulting composite structure to convert said normal magnesium carbonate to basic magnesium carbonate, thereby forming a composite heat insulation comprising a slab of low temperature heat-insulation material integral with a slab of high temperature heat-insulation material.

2. A method of making a heat insulation struci5 ture which comprises making a slab of heat insulation material in a filter mold by forcing an aqueous slurry of solid heat-insulation ingredients into the mold with drainage of water from the mold. removing the formed slab from the filter mold, then drying and trimming said slab, then molding in integral contact with a surface of said heat insulation material and in the form of a slab an aqueous slurry comprising normal magnesium carbonate and a filler, and then heating the composite structure while said molded slurry is moist to convert said normal magnesium carbonate to basic magnesium carbonate.

3. A method of making a heat insulation which comprises making a slab of heat insulation material which slab is of the predetermined ultimate dimensions desired in the heat-insulation product and is of substantial strength, making an aqueous slurry comprising heat insulation ingredients, said slurry being adapted to become set by chemical action to a rigid mass, molding said slurry in integral contact with a surface of said slab of heat insulation material, said slurry thereafter setting and being dried to form a composite heat insulation comprising two integrally united slabs of heat insulation material having diiferent heat resistance characteristics.

4. A method according to claim 3 wherein said slurry is adapted to set hydraulically and after molding becomes hydraulically set.

5. A method according to claim 3 wherein said slurry is adapted to set upon being heated and after molding is set by application of heat.

6. A method according to claim 3 wherein the slab of heat insulation material that is first made is made in a filter mold by forcing an aqueous slurry of solid heat-insulation ingredients into the mold with drainage of water from the mold and is removed from the mold, dried, and trimmed to the desired predetermined dimensions before molding said slurry in integral contact therewith.

7. A method, according to claim 3, wherein said first-formed slab has mechanical securing means embedded therein and protruding from a face thereof, and said other slab is molded in integral contact with said face, thereby causing said mechanical securing means protruding from said first slab to become embedded in said second slab.

8. A method, according to claim 3, wherein the slab molded from said slurry is molded integrally with the other slab in offset relation there- 9. A method of making a heat insulation which comprises making a first slab of heat-insulation material in a filter mold by forcing an aqueous slurry, of solid heat-insulation ingredients into the mold with drainage of water from the mold, removing said slab from the mold, drying the slab, and trimming said slab to desired dimensions, making a slurry comprising heat-insulation ingredients, said slurry being adapted to become set by chemical action to a rigid mass, molding said slurry in integral contact with a surface of said dried and trimmed first slab in the form of a second slab integral therewith, said slurry thereafter setting and being dried, thereby forming a composite heat-insulation material comprising two integrally-bonded slabs of heat-insulation material having diiferent heat-resistance characteristics.

10. As an article of manufacture, a preformed heat-insulation structure comprising a first slab oi. heat insulation material deficient in resistance to temperatures above 650 F. and integrally Joined race to face therewith, a second slab oi heat insulation material resistant to temperatures substantially above 550 F., at least one or said slabs containing a major proportion by weight of basic magnesium carbonate and being rigidly set with the particles 01' said basic magnesium carbonate that constitute the major proportion by weight oi said slab in the interbonded relationship that results from the production of said basic magnesium carbonate by in situ decomposition ot normal magnesium carbonate to iorm said basic-magnesium carbonate and the juncture between said slabs being characterized by the cemented relationship of the basic magnesium carbonate at the surface of the slab containing a major proportion by weight of basic magnesium carbonate with the surface oi the heat insulation material of the other slab, which cemented relationship results from said in situ decomposition oi the normal magnesium carbonate intimately contacting the material of said other slab at said juncture.

11. as an article of manufacture. a preformed heat-insulation structure comprising a ilrst slab of heat insulation material and integrally Joined face to face therewith, a second slab of heat insulation material having substantially diilerent heat-resistance characteristics from said first slab, at least one or said slabs containing basic magnesium carbonate and being rigidly set with the particles of said basic um carbonate in the inter-bonded relationship that results from the production of said basic magnesium carbonate by in situ decomposition of normal msgnesium carbonate to form said basic magnesium carbonate, and the juncture between said slabs being characterized by the cemented relationship 0! the basic magnesium carbonate in the slab containing the same with the surface .0! the heat insulation material of the other slab. which cemented relationship results from said in situ decomposition 01' normal magnesium carbonate intimately contacting the material of said other slab at said juncture.

ROGER A. MAcAR'I'HUR. HAROLD W. GRIIDIR. 

